High temperature furnace



Feb. 4, 1964 s. MALOOF ETAL 3,120,597

HIGH TEMPERATURE FURNACE 2 Sheets-Sheet 1 Filed Sept. 27. 1961 INVENTORS LEON S. MALOOF FIG. 2

BY LOUIS KOPITO WM 1' M ATTORNEYS 4, 1964 1.. s. MALOOF ETAL 3, 0, 97

HIGH TEMPERATURE FURNACE 2 Sheets-Sheet '2 Filed Sept. 27. 1961 INVENTORS LEON S. MALOOF BY LOUIS KOPITO Wm v Cw riuu ATTORNEYS;

United States Patent 3,120,597 HIGH TEMPERATURE FURNACE Leon S. Maloof, Watertown, and Louis Kopito, Brookline, Mass, assignors to Baird Atomic, Inc., Cambridge, Mass, a corporation of Massachusetts Filed Sept. 27, 1961, Ser. No. 141,103 13 Claims. (Cl. 219-19) This invention relates in general to high temperature furnaces and more particularly concerns improvements in furnaces of the sort that employ electrically energized graphite cloth as the heating element.

In a co-pending application entitled High Temperature Furnace, Serial No. 105,152 and filed April 24, 1961, there is disclosed a device for generating very high temperatures by applying an electric current across a section of graphite cloth. In the above application, a single ply of graphite cloth, in strip or rectangular form, is supported between two sets of carbon gripping elements which are rotatably mounted on cooling tubes. These tubes form part of the electrical circuit by which the cloth is energized.

The gripping elements used to support the cloth heretofore have been shaped into cylindrical cams and tubular blocks which are extremely difiicult to manufacture since they must be held to very close tolerances in order to provide a proper gripping pressure for any one thickness of cloth. Also, the cloth mounting arrangement has been somewhat awkward to operate since the rollers must be manually adjusted each time a section of cloth is replaced. Accordingly, it is an object of the present invention to provide in a high temperature furnace having a graphite cloth heating element means for quickly and conveniently replacing the element.

Another object of this invention is to provide a high temperature furnace in which the bite between a pair of carbon electrodes may be readily enlarged to receive a section of graphite cloth inserted therebetween and then closed to hold the cloth under a uniform gripping pressure during wide changes in temperature.

Yet another object of this invention is to provide an improved graphite cloth heating element for use in high temperature furnaces.

Still another object of this invention is to provide an improved combination cooling system and electrical circuit for operating a high temperature furnace.

More particularly this invention features a furnace having a graphite cloth heating element mounted between sets of quick-clamping carbon electrodes and capable of attaining very high temperatures within a brief period of time when electrically energized. The electrodes are connected to an operating lever whereby the bite of both sets of electrodes may be opened or closed simultaneously by simply biasing the lever handle. 1

As another feature of this invention, several plies of graphite cloth sections have been cemented together in face to face contact to form a laterally stable heating element which may be easily inserted between the carbon electrodes. Such an element possesses the further advantage of having a radiating surface that is much greater than that of a single layer of cloth of the same dimensions.

But these and other features of the invention, along with further objects and advantages thereof, will become more readily apparent from the following detailed de scription taken in connection with the accompanying drawings in which:

FIG. 1 is a view in perspective of a high temperature furnace made according to the invention;

FIG. 2 is a top plan view of the electrode assembly featured in the invention;

FIG. 3 is a view in side elevation of the assembly;

FIG. 4 is a diagrammatic representation of the electrode cooling system;

FIG. 5 is a diagram of the circuit used to operate the furnace;

FIG. 6 is a perspective view showing a modification of the carbon electrodes shown in FIGS. 1-3;

FIG. 7 is a view similar to FIG. 6 but showing a further modification of the electrodes; and

FIG. 8 is a perspective view of a graphite cloth heating element in which several plies have 'been cemented together.

By way of a general description, there is illustrated in the drawings a high temperature furnace organized about an improved electrode clamping assembly indicated by the reference character 10 and adapted to grippingly sup port and provide electrical connections to a section of graphite cloth 12 which serves as the heating element for the furnace when electrically energized. The clamping assembly 10 is supported on a cabinet base 14 which may be used to house various accessories as well as to provide a platform for the furnace. Preferably, the top panel section 16 of the cabinet should be fabricated from a thermal insulating material such as asbestos, since the furnace is capable of generating temperatures ranging up to 6600 degrees -F. A removable, transparent dome 18 is also provided for enclosing the cloth 12 and clamping assembly 10 within a sealed chamber 20. Conduits 17 and 19 are provided for introducing an inert gas into the chamber 20 if desired. The dome 13 may be composed of any suitable transparent vitreous material such as Pyrex, Vycor or quartz.

A preferred graphite fabric is plain, woven or felted from fibers of high purity graphite carbon having a tensile strength from 50,000 to 100,000 p.s.i. The fibers themselves may be produced by processing carbonaceous materials at temperatures up to 5400 degrees F. Such graphite fibers display high thermal conductivity and good electrical conductivity. Chemical analysis of these fibers indicate a composition of approximately 99.09% plus carbon and approximately 0.04% ash. No major impurities appear and the minor impurities are limited to magnesium and aluminum with traces of calcium, iron, manganese, silicon, boron, copper, nickel, and sodium. The graphite material will sublime at approximately 6600 degrees F. (3650 degress C.) without melting. The average diameter of the filaments ranges from 0.00005 to 0.001 inch. The woven fabric has a thread count per inch ranging from 20 to 30, a gauge ranging from 0.01 to 0.09 and number of filaments per ply ranging from 1 to 2,000.

In practice the section of graphite cloth 10 is supplied with from 200 to 15,000 watts per square inch of fabric in order to produce a temperature in excess of 3000 degrees C. within approximately one second.

Referring now more particularly to FIGS. 1, 2 and 3 of the drawings, the improved electrode clamping assembly 10 will now be described in detail. The graphite cloth 12, as shown, has a generally rectangular configuration with opposite ends supported by two spaced electrodes 22 and 24. Each electrode includes an upper tubular cylindrical element 26 and a lower tubular cylindrical element 28, both of carbon. These elements are supported on opposite legs of a U-shaped cooling conduit 30, formed from A tubing and extending from the front face of metal block '32. As indicated in FIG. 3, the block 32 is mounted on an insulating base plate 33 and is formed with internal passages 34 and 36 for delivering a flow of cooling liquid through the conduit 3%.

It will be noted in FIG. 3 that the upper carbon element 26 has an inside diameter that is approximately equal to the outside diameter of the conduit 30, whereas the inside diameter of the lower element 28 is considerably larger, three eighths of an inch, for example, so that the lower element has a one eighth inch clearance and fits quite loosely over the lower leg of the conduit 31). The arrangement and dimensions of the carbon elements 26 and 28 are such that the two elements form a bite which may be readily opened or closed by lowering or raising the lower carbon element 28. T this end a lever arm 38 is pivotally mounted by means of a pin 40 to each of the blocks 32.. The upper extremity of the two lever arms are connected by a cross bar 42 which serves as a common handle so that both lever arms may be pivoted simultaneously. The lower extremity of each lever arm is provided with a ceramic boss at capable of withstanding extremely high temperatures. The boss extends under the lower carbon element 28 and contacts it at its midpoint. A tensioned coil spring 46 connected between the blocks 32 and the lever arm 38 provides a counterclockwise moment on the arm so that ceramic boss 44 presses up against the carbon element 28 to hold it in a normally upward or closed position.

It will be readily understood that a section of graphite cloth =12 may be quickly and conveniently clamped between the two electrodes ZZI and 24 by simply biasing the handle 42 upwards so as to permit the lower carbon element 2% to drop down and onto the lower leg of the conduit 39. The graphite cloth may then be slipped edgewise through the open bite of both electrodes. Once the cloth is in position, the handle is rel-eased and the bite is closed by reason of the spring 46 biasing the lever 38.

In order to insure an even gripping pressure by both electrodes, the cross bar 42 should connect rather loosely with the levers 38. The resulting play will permit each of the levers to operate somewhat independently of each other. If the two levers were rigidly connected, any misalignment of parts might prevent one of the electrodes from fully closing.

It will be noted in FIG. 3 that the tension spring 46 is located to the rear of the block 32 and as far away from the cloth 12 as possible. In this position, the springs are completely shadowed by the blocks 32 and will not be subjected to the intense radiation emitted by the cloth during energization. By locating the springs out of the path of radiation, their elastic characteristics will not be impaired despite frequent use of the furnace.

In FIGS. 6 and 7 of the drawings, there are illustrated .two modifications in the configuration of the tubular carbon elements which serve as the electrodes. In FIG. 6, the electrode is shown as including an upper tubular cylindrical element 48, which is identical to the element 26 of FIG. 3, and a lower tubular element d of rectangular cross-section. As before, both elements are formed with axial bores 52, 54 with the upper bore dimensioned to fit snugly over one leg of the cooling conduit, while the lower bore 54 is somewhat enlarged so that the lower element may be moved to or away from the upper element. Also the lower element has a transverse recess 56- formed midway between its ends and adjacent to the bottom edge. The recess 56 accommodates the ceramic boss 44 of the lever arm 33 so that a positive opening action of the d electrodes is available when the arm is biased to the open position.

In FIG. 7, the electrode is shown as having upper and lower eiements 58 and 60 both. of annular cross-section. The upper element is formed with an enlarged midportion and tapers to reduced end portions. The lower element, on the other hand, is shaped with enlarged end portions and a reduced mid-portion to mate with the upper element. As before, the lower element has a rather large axial bore to permit limited movement to or away from the upper element. With this latter configuration, a section of graphite cloth clamped in position will assume a longitudinal trough so that a specimen placed therein will not roll or fall off the cloth.

Referring now to FIG. 4, there is illustrated animproved cooling system for the carbon electrodes 22 and 24. In this system, a flow of cooling water is delivered from a suit-able source through an inlet 62 and passed serially through a flow indicator 64-, then through the upper element 26 and back through the lower element 28 of the electrode 24-. From there the conduit makes a wide loop 66 to feed the water through the upper element of the electrode 26, back through the lower element and then through an outlet 68. With a cooling system of this sort, blockage anywhere in the system will show up immediately on the flow indicator 64.

The cooling conduit as shown in FIG. 4 serves as part of the electrical circuit for energizing the graphite cloth. Electrical leads 7t} and 72" connect the conduit to a variable power source 74. By forming the large loop 66 in the cooling conduit, a very high electrical resistance is formed which prevents current leakage and causes all of the applied current to appear across the graphite cloth 112 which has a very small resistance. In the circuit diagram of FIG. 5, the loop 66 is represented by the resistor 76 and the graphite cloth represented by the resistor 78.

Also shown in the FIG. 5 circuit is a lamp 550 which has a resistance intermediate between that of the cloth and that of the loop. The function of this lamp is to serve as a warning light for the operator. With this circuit, the lamp will not operate until the graphite cloth 12 has burned out. As long as the circuit is energized, the lamp will continue to glow and indicate to the operator that he should avoid touching the electrodes until the circuit has been opened and the electrodes allowed to cool.

In practice, however, there is normally no need to touch the electrodes since the graphite cloth may be readily mounted between the electrodes without the operators hands contacting the electrodes. By merely biasing the handle 42, the bites between the carbon elements of both electrodes are opened and the free ends of the graphite cloth may be readily inserted. Once in position the handle is released and the furnace is ready to be energized.

In order to further facilitate changing of graphite cloth sections and to provide a more eliective source of heat, the graphite cloth element 12 is fabricated from a number of single plies of fabric bonded together, as by cement, in face to face contact. The resulting product, as shown in FIG. 8 is a laterally stable element having an effective radiation surface many times greater than that of a one ply section of cloth of the same thickness. Graphite cloth, when woven in thin sheets, is very soft and flexible and therefore is somewhat awkward to handle when trying to feed it into the bite of the electrodes. By bonding several layers together, the element is stiffened and is much easier to manipulate.

While the invention has been described with particular reference to the illustrated embodiments, it will be apparent that numerous modifications will appear to those skilled in the art.

Having thus described our invention, what we claim as new and desire to obtain by Letters Patent of the United States is:

1. An electric heating device, comprising a base, a pair of serially connected coolant conduits mounted on said base, each of said conduits forming a U shaped loop having two parallel legs one of which is arranged vertically above the other, a tubular carbon element mounted over each of said legs and forming a bite therebetween, the lower of said carbon elements being formed with an axial opening of somewhat greater diameter than that of its supporting leg whereby said lower carbon element may be moved into or out of contact with the upper carbon element, spring loaded lever means mounted on said base and engaging with each of said lower carbon elements for urging said elements into contact with said upper elements, the carbon elements associated with each of said conduits constituting an electrode, a section of graphite fabric mounted in the bite of the upper and lower carbon elements of both of said electrodes and means for applying electrical power between said electrodes.

2. An electric heating device, comprising a base, a pair of coolant conduits mounted on said base, each of said conduits forming a U-shaped loop having two parallel egs one of which is arranged vertically above the other, a tubular carbon element mounted over each of said legs and forming a bite therebetween, the lower of said carbon elements being formed with an axial opening of somewhat greater diameter than that of its supporting leg whereby said lower carbon element may be moved up into or down out of contact with the upper carbon element, biasing means mounted on said base and engaging with each of said lower carbon elements for urging said elements into contact with said upper elements, the carbon elements associated with each of said conduits constituting an electrode, a section of graphite fabric mounted in the bite of the upper and lower carbon elements of both of said electrodes and means for applying electrical power between said electrodes.

3. An electric heating device, comprising a base, a pair of coolant conduits mounted on said base, each of said conduits forming a U-shaped loop having two parallel legs one of which is arranged vertically above the other, a tubular carbon element mounted over each of said legs and forming a bite therebetween, the upper of said carbon elements snugly engaging its supporting leg and the lower of said carbon elements being formed with an axial opening of somewhat greater diameter than that of its supporting leg whereby said lower carbon element may be moved into or out of contact with the upper carbon element, spring biasing means mounted on said base and engaging with each of said lower carbon elements for normally urging said elements into contact with said upper elements, the carbon elements associated with each of said conduits constituting an electrode, a section of graphite fabric mounted in the bite of the upper and lower carbon elements of both of said electrodes and means for applying electrical power between said electrodes.

4. An electric heating device according to claim 3 wherein said carbon elements are cylindrical.

5. An electric heating device according to claim 3 wherein the upper carbon elements are formed with enlarged center portions and the lower carbon elements are formed with restricted center portions whereby a longitudinal trough is imparted into said fabric when mounted between said electrodes.

6. An electric heating device according to claim 3 wherein said lower carbon element is rectangular in crosssection.

7. An electric heating device, comprising a base, a pair of serially connected coolant conduits mounted on said base, each of said conduits forming a U-shaped loop having two parallel legs one of which is arranged vertically above the other, a tubular carbon element mounted over each of said legs and forming a bite therebetween, the lower of said carbon elements being formed with an axial opening of somewhat greater diameter than that of its supporting leg whereby said lower carbon element may be moved into or out of contact with the upper carbon element, a pair of levers mounted on said base and engaging with each of said lower carbon elements, spring means biasing said levers and urging said lower elements into contact with said upper elements, the carbon elements associated with each of said conduits constituting an electrode, a section of graphite fabric mounted in the bite of the upper and lower carbon elements of both of said electrodes and means including said conduits for applying electrical power between said electrodes.

8. An electric heating device according to claim 7 including means for shielding said spring means from said fabric.

9. An electric heating device, comprising a base, a pair of conduits mounted on said base, each of said conduits forming a U-shaped loop having two parallel legs one of which is arranged vertically above the other, a tubular carbon element mounted over each of said legs and forming a bite therebetween, the lower of said carbon elements being formed with an axial opening of somewhat greater diameter than that of its supporting leg whereby said lower carbon element may be moved into or out of contact with the upper carbon element, a pair of levers mounted on said base and engaging with each of said lower carbon elements, spring means biasing said levers and urging said lower elements into contact with said upper elements for closing said bite, handle means connecting said lovers to manually counter bias said levers t o open said bite, the carbon elements associated with eaEh of said conduits constituting an electrode, a section of graphite fabric mounted in the bite of the upper and lower carbon elements of both of said electrodes and means for applying electrical power between said electrodes.

10. An electric heating device, comprising a base, a pair of electrode supports mounted in spaced relation on said base, each of said supports having two parallel legs one of which is arranged vertically above the other, a tubular carbon element mounted over each of said legs and forming a bite therebetween, the lower of said carbon elements being formed with an axial opening of somewhat greater diameter than that of its supporting leg whereby said lower carbon element may be moved into or out of contact with the upper carbon element, a pair of levers mounted on said base and engaging with each of said lower carbon elements, means urging said elements into contact with said upper elements to close said bite and means connecting both of said levers to manually counter bias said levers to open said bite, the carbon elements associated with each of said conduits constituing an electrode, a section of graphite fabric mounted in the bite of the upper and lower carbon elements of both of said electrodes and means for applying electrical power between said electrodes.

11. In a heating device employing a section of graphite fabric mounted between a pair of carbon electrodes, a circuit for energizing said fabric, comprising a cooling conduit serially connecting said electrodes, said conduit forming between said electrodes a loop having an aeotrical resistance substantially higher than that of said fabric, a variable power source connected to said conduit and a signal lamp connected to said conduit and having an electrical resistance intermediate to that of said fabric and said loop.

12. Apparatus for supporting a section of graphite fabric, comprising a base, spaced electrode assemblies mounted on said base for engaging spaced sections of said fabric, each of said assemblies including jaw portions mounted for movement relative to one another, at least one of said portions being formed from carbon, means normally biasing said jaw portions towards one another for clamping engagement of said fabric and means connecting said biasing means for manually counter-biasing said biasing means.

13. Apparatus according to claim 12 including conduit 7 9 as means extending through said jaw portions for passing a 2,868,942 Lyijynen Jan. 13, 1959 cooling medium through said electrode assemblies. 2,930,879 Scatchard Mar. 29, 1960 2,978,666 McGregor Apr. 4, 1961 References Citedinithe file of this patent 2,981,818 Trabiley Apr. 25, 1961 UNITED STATES PATENTS 5 2 y 23, 1961 L 1,401,301 Baldwin M Dec 27, 1921 Albert 94 l 111m 1962 1,741,882 Robinson Dec. 31, 1929 FOREIGN PATENTS gggiggj ipalding 2 3;; 715,227 Great Britain Sept. 8, 1954 anger ec. 2,822,575 Imbert et a1. Feb. 11, 1958 10 OTHER REFERENCES 2,859,322 Glazier et al. Nov. 4, 1958 Metal Progress, May 1959, pages '115116. 

11. IN A HEATING DEVICE EMPLOYING A SECTION OF GRAPHITE FABRIC MOUNTED BETWEEN A PAIR OF CARBON ELECTRODES, A CIRCUIT FOR ENERGIZING SAID FABRIC, COMPRISING A COOLING CONDUIT SERIALLY CONNECTING SAID ELECTRODES, SAID CONDUIT FORMING BETWEEN SAID ELECTRODES A LOOP HAVING AN ELECTRICAL RESISTANCE SUBSTANTIALLY HGIHER THAN THAT OF SAID FABRIC, A VARIABLE POWER SOURCE CONNECTED TO SAID CONDUIT AND A SIGNAL LAMP CONNECTED TO SAID CONDUIT AND HAVING AN ELECTRICAL RESISTANCE INTERMEDIATE IN THAT OF SAID FABRIC AND SAID LOOP. 